Several applications depend upon high current switching ability, high dielectric breakdown voltage, high vibration and shock resistance, high reliability, extreme cleanliness or low contamination particle count, and narrow temperature differential between the open and closed switch positions. For example, qualification for many space and military applications requires a 5 ampere, 28 volt D.C. switching capability capable of 100,000 cycles and no internal particles measuring 0.001 inch or larger which may become lodged between the switch contacts and cause an open condition. Temperature differential is measured as the number of degrees above or below the switch set point where the bi-metal actuator disc reverses state and thereby reverses the open/closed condition of the switch. Temperature differential is often required to be quite narrow, for example, on the order of 1 degree Centigrade or less.
Snap acting thermal switches are presently being used. Snap acting bi-metal disc-type thermal switches typically have a contact movably mounted on a carrier with the movement of the carrier controlled by a bi-metal actuator disc. The bi-metal disc actuates the switch by changing from a convex state to a concave state at a temperature set point dependent upon the difference in thermal expansion coefficients of the two materials forming the bi-metallic disc. The bi-metal actuator disc alternates between a convex state and a concave state as the ambient temperature rises above or drops below the switch set point. The change in state exerts force on the movable carrier to open the contacts or relieves the force to close the contacts. The movable carrier is typically a spring, for example, a leaf spring, commonly referred to as an armature, which tends to force the switch movable contact against a stationary contact to close a circuit. The armature is typically an electrically conductive current carrying member of the switch circuit. The actuating movement of the bi-metal disc is coupled to the contact mechanism through an insulated coupling pin or plunger commonly referred to as a striker pin which is fastened in fixed relation to the movable carrier.
The spring rate or spring force of the contact carrier spring or armature is instrumental in determining the switch closing set point. The armature spring holds the contacts closed when the bi-metal actuator disc is not engaged with the striker pin. When the bi-metal actuator disc changes state to force the switch contacts into an open position, spring force is exerted against the bi-metal actuator disc by the armature spring acting through the striker pin. Thus, when the contacts are in the open position, the armature spring exerts force on the bi-metal actuator disc tending to force the bi-metal disc to change its convex/concave state. Thus, the armature spring force affects the temperature at which the disc changes its convex/concave state by supplying extra force needed to overcome hoop stress in the disc during the transition between the convex and concave states. The armature spring force is typically adjusted into a narrow range of spring forces by deforming the armature itself either toward the bi-metal actuator disc to increase spring force or away from the bi-metal actuator disc to decrease spring force. Deformation of the armature introduces stresses into the armature spring which lead to switch instability as the stresses relieve over time and thermal cycling. As the stresses relieve, effective armature spring force changes. Changes in effective armature spring force results in thermal drift of the switch set point.
This striker pin is normally formed of a vitreous material, for example, ceramic, alumina or steatite. The length of the striker pin must be precisely controlled to properly couple the snap travel of the bi-metal disc to the contacts. Improper striker pin lengths result in improper switch action and either gross reduction in switching life or susceptibility to intermittent contact closings during vibration. Normal manufacturing tolerances do not allow the striker pin length to be controlled directly without extraordinarily tight controls on the several components that make up the assembly. As a result, normal practice has been to manufacture the detail components to common tolerances and compensate for the total accumulation of plus and minus tolerances by using a striker pin fitted to each specific assembly. Several common methods are now used to fit the striker pin length to each switch assembly. Each have imitations and disadvantages.
One commonly used current method utilizes a free-floating coupling pin, manufactured in incremental lengths to cover all possible combinations of tolerance accumulations. Each switch-contact assembly is measured using specialized gauges which relate the geometry of each assembly to a specific pin size. The specified pin length is selected from available stock and installed in the switch. Since this design approach does not fix the striker pin to any support, it is free to rattle and bounce within the enclosure whereby contamination from rubbing surfaces can be generated. Vibration and shock exposures can also impact the floating striker pin against the contact assembly thereby causing inadvertent openings or closings of switch contacts. Fractures of the pin as a result of extreme shock and vibration levels have been observed in switches using the floating striker pin approach.
Another commonly used procedure for obtaining correct pin length is mechanically attaching a pin of sufficient length to compensate for all combinations of component part tolerances to a fixed part of the assembly and trim the point or lower end to the specific dimension required. This procedure provides superior resistance to high vibration and shock levels because no "loose" parts are in the disc-to-contact train. However, the trimming operation inherently creates debris in the form of chips or grindings which have the potential for contaminating switch contacts. Elaborate procedures are often required to thoroughly clean the switch assembly.
Furthermore, in grinding the striker pin to length, a sharp-edged, flat tip or lower end is formed which results in harmful abrasive wear of the actuating bi-metal disc by repeated contact therewith. Additionally, the sharp edge left by the grinding operation tends to chip whereby chips break off during operation to cause contamination within the finished switch assembly.
Yet another procedure for obtaining proper striker pin length is described in U.S. Pat. No. 4,201,967 ('967). The procedure of '967 provides a striker pin of ceramic material bonded to a carrier by and adhesive layer of controlled thickness for establishing the effective length of the striker pin. The patent also discloses a method of manufacture including a tool used therein. The procedure of '967 overcomes some of the problems of the prior art by providing a spherical lower end which does not require grinding. However, this procedure is only accomplished by using tedious and time-consuming assembly techniques.
Still another procedure for obtaining proper striker pin length utilizes a fixed, pre-formed striker pin with a cap adjustably fitted thereon. In this procedure, a cup-shaped metal cap is mounted onto the lower end of the striker pin using a small layer of adhesive between the striker pin and the cap. This procedure also overcomes some of the problems of the prior art by simplifying the tooling and assembly techniques required. However, this procedure lowers the switch's operational vibration and shock environmental limits because the striker pin cap and adhesive layer increase the mass of the striker pin. The spring constant of the movable mount or armature on which the contact and striker pin are mounted must be increased to overcome the increased mass of the striker pin cap and adhesive and prevent contact chatter. Switch performance is degraded because the bi-metal actuator disc must overcome the greater spring force and separate the contacts. For example, the increased actuator strength required to overcome the greater spring force increases the temperature differential between the concave and convex states of the bimetallic actuator disc, effectively increasing the overlap between the switch's open and closed positions. Switch dielectric strength is degraded because the electrically conductive metal striker pin cap reduces the effective insulated path between the actuating bi-metal disc and the electrically conductive spring mount. Furthermore, sputter coating of the insulating portion of the striker pin during make and break operation of the contacts over repeated cycling reduces the insulation resistance of the circuit.
One more procedure for obtaining proper striker pin length is accomplished by providing a fixed length striker pin and adjusting the striker pints length relative to the bi-metal actuator disc by deforming either or both of the armature spring and the stationary contact. This procedure induces stresses into the armature spring which lead to switch instability as the stresses relieve over time and thermal cycling. As the stresses relieve, effective striker pin length and armature spring force change. Changes in either or both of effective striker pin length and armature spring force result in thermal drift of the switch set point and increased contact chatter during opening and closing of the contacts. Furthermore, deforming the stationary contact degrades structural integrity of its mechanical mount to its support structure with unpredictable results.
Failing to use one of the above procedures to obtain a proper striker pin length: matching a striker pin to a specific assembly; trimming the striker pin in the assembly by grinding; adjusting the effective striker pin length with a layer of adhesive; providing a striker pin cap adhered to the striker pin with a layer of adhesive; and deforming either or both of the armature spring and the stationary contact, renders the snap acting thermal switch inoperative either because the striker pin is too long to allow the contacts to close or too short for the bi-metal actuator disc to open the contacts. However, as discussed, each procedure has drawbacks. Therefore, a procedure overcoming the limitations of the prior art is desirable.